1. Field of the Invention
The present invention relates to a safety apparatus for an output level-adjustable microwave oven, and more particularly, to a safety apparatus for an output level-adjustable microwave oven that can prevent a fire caused by defects in relays that adjust an output level of the microwave oven.
2. Description of the Related Art
Generally, a microwave is an apparatus which cooks food using microwaves, and is provided with a high voltage transformer and a magnetron. The high voltage transformer operates the magnetron by generating a certain high voltage. The magnetron emits the microwaves of a high frequency, for example, about 2450 MHz, in a space where the food is deposited. The emitted microwaves vibrate water molecules contained in the food so as to heat and cook the food by a frictional heat generated by the vibration of the water molecules.
FIG. 1 shows a circuit diagram of a conventional output level-adjustable microwave oven. Here, the output level of the conventional microwave oven is adjusted by adjusting a number of windings of a primary coil of a high voltage transformer.
As shown in FIG. 1, first and second commercial Alternating Current (AC) lines L1 and L2 are connected at their one terminals to respective input terminals of a noise filter 1. The first and second commercial AC lines L1 and L2 supply 220V commercial Alternating Current (AC) power to a noise filter 1, and are connected at their other terminals to both terminals of a primary coil of a high voltage transformer HVT. The noise filter 1 includes elements, such as first and second fuses FUSE1 and FUSE2, capacitors C1 to C3, an inductor L, and a resistor R1.
The first fuse FUSE1 is connected to the first commercial AC line L1, and the second fuse FUSE2 is connected between the second commercial AC line L2 and a monitor switch SW2 (described hereinbelow).
First and second temperature switches TCO1 and TCO2, a door switch SW1 and the monitor switch SW2 are connected in series to the first commercial AC line L1. A high output relay RY1, a low output relay RY2 and an inrush relay RY3 are connected in parallel to the second commercial AC line L2.
The first temperature switch TCO1 is turned on or off according to a temperature of a magnetron MGT, so as to prevent overheating of the magnetron MGT. The second temperature switch TCO2 is turned on or off according to a temperature of a cooking chamber (not shown), so as to prevent overheating of the cooking chamber. The door switch SW1 and the monitor switch SW2 perform a safety function by being turned on or off according to an open or a closed state of a door (not shown). A lamp 2, a fan motor 3 and a drive motor 4 are connected between the first and second commercial AC lines L1 and L2.
A center tap A is provided midway between the terminals of the primary coil. The high voltage transformer HVT is connected at its center tap A to the high output relay RY1 through a first power supply line E1, and the high output relay RY1 is turned on or off by a microcomputer 7. The high voltage transformer HVT is connected at its one terminal to the low output relay RY2 through a second power supply line E2, and the low output relay RY2 is turned on or off by the microcomputer 7.
The inrush relay RY3 is operated for several minutes just before the low output relay RY2 is turned on, so as to remove an inrush current.
A secondary coil of the high voltage transformer HVT is connected to the magnetron MGT, a high voltage condenser HVC, a resistor R3, a high voltage diode HVD and a high voltage fuse H.V.FUSE.
The microcomputer 7 is connected to a display unit 5 and an input unit 6. The microcomputer 7 receives cooking commands of a user, such as a command to set a cooking menu and a start command, from the input unit 6. The display unit 5 displays various kinds of operational states, such as the setting of the cooking menu and cooking states of the microwave oven.
The microcomputer 7 drives the magnetron MGT in response to a cooking start command. Where it is required to generate a high output of 1100 W from the magnetron MGT, the microcomputer 7 controls the high output relay RY1 and the low output relay RY2 to be turned on and off, respectively. On the other hand, where it is required to generate a low output of 1000 W from the magnetron MGT, the microcomputer 7 controls the high output relay RY1 and the low output relay RY2 to be turned off and on, respectively.
However, the conventional microwave oven is problematic in that a fire may occur due to the defects in the relays RY1 and RY2, which adjust an output of the microwave oven, and there is no scheme to deal with such a hazard. That is, where any one of the relays RY1 and RY2 has melted due to the application of the inrush current, both the relays RY1 and RY2 may be simultaneously turned on in a process of adjusting the output of the microwave oven. In this case, an excessive current flows through the power supply lines E1 and E2, which may generate a high temperature heat and burn covering materials that shield the power supply lines E1 and E2. Accordingly, surrounding electrical parts may be damaged through a resulting fire.